The conventional ball spline unit of this type has a structure comprising a cylindrical outer cylinder to which a plurality of loaded ball rolling grooves are formed at an inner periphery of the outer cylinder. This outer cylinder is provided with a plurality of unloaded ball passages corresponding to the loaded ball rolling grooves. Both ends portions of the outer cylinder are provided with side covers each having a direction changing passage for communicating the loaded ball rolling grooves with the unloaded ball passages. Further, in order to prevent balls disposed in the loaded ball rolling grooves from dropping off, retainer portions are provided with the inner periphery of the outer cylinder along the loaded ball rolling grooves.
A spline shaft is inserted into the outer cylinder so that the outer cylinder is reciprocally moved. At an outer periphery of the spline shaft, there are formed with loaded ball rolling grooves corresponding to the loaded ball rolling grooves formed to the outer cylinder, and a number of balls are disposed between the loaded ball rolling grooves.
However, in the conventional spline unit described above, since a radius of curvature of the direction changing passage is small, sticks of the ball are liable to occur. In order to prevent the sticking of the balls, it is effective to increase the radius of curvature of the direction changing passage. However, such structure will result in increase of a size of the spline unit.
On the other hand, it has been conventionally attempted to lower a noise level and manufacturing cost of the spline unit by forming ball supporting portions, side covers and the unloaded ball passages as resin molded members for which high rigidity is not required.
However, each of the conventional resin molded members was a separately molded member which is separately formed form the outer cylinder. Therefore, an assembling process for integrating the molded members was required after completion of molding the respective members.
In this regard, one may happen to think of integrally molding the ball supporting portions or the like with the outer cylinder for the purpose of reducing the assembling steps.
As an example of such integrally molding technique, for example, a method of forming a linear motion ball bearing shown in U.S. Pat. No. 4,128,279 is well known.
As shown in FIG. 9, the aforementioned linear motion ball bearing is constructed so as to comprise an outer cylinder 101 a part of which is cut out so as to have an opened sectional shape, and a shaft 105 to be inserted into the outer cylinder 101.
A number of balls 104 arranged in an axial direction are disposed between the inner circumference of the outer cylinder 101 and the outer circumference of the shaft 105, so that the outer cylinder 101 can linearly move along the shaft 105. The balls 104 are arranged in a circumferential direction of the outer cylinder 101 so as to form a plurality of ball rows. Each of the ball rows is arranged so as to circulate through the direction changing passages 108 formed to both ends of the outer cylinder 101 and through the plurality of unloaded ball passages 110 provided to the outer circumference of the outer cylinder 101. The balls 104 disposed in the loaded region between the outer cylinder 101 and the shaft 105 are retained by retainer portions 106 provided at both sides of the ball row so as to extend along the axial direction.
Further, afore-mentioned retainer portions 106, inner peripheral portion 108a of the direction changing passage 108 and the unloaded ball passages 110 were integrally molded with the outer cylinder 101 by using an insert-molding method.
However, in a case of the conventional linear motion ball bearing described above, when the insert molding is performed, as shown in FIG. 9(c), for example, the outer periphery of the outer cylinder 101 is tightly contact to a first molding die 111 while the inner periphery of the outer cylinder 101 is tightly contact to a second molding die 112 thereby to form cavities 106a and 110a for molding the retaining portions 106 and the unloaded ball passages 110. However, it was difficult for the inner and outer peripheries of the outer cylinder 101 to accurately and tightly contact to the first and second cavities 111 and 112, so that there was arisen a problem that a gap was liable to occur between the contacting surfaces, generating burrs.
In particular, when the burrs are generated between the contacting surfaces of the inner periphery of the outer cylinder 101 and the second molding die 112, thus resulting in generation of the burrs at loaded ball rolling surfaces, so that such burrs are required to be removed. However, it is very difficult and practically impossible to remove such burrs generated at inner portions between the retaining portions 106 and 106.
When such burrs exist, the circulation of the balls 104 is obstructed, so that a feeding accuracy is deteriorated. In the worst case, the balls 104 are jammed or clogged up to stop a machine using the spline unit, thus resulting in serious influence on productivity.
The present invention has been conceived for solving the afore-mentioned problems encountered in the prior arts, and an object of the present invention is to provide a ball spline unit enabling the balls to smoothly circulate without increasing the size of the ball spline unit.
Further, another object is to provide a method of molding an outer cylinder of the ball spline unit which is capable of being integrally molded by accurately setting positions of resin portions to be formed to the outer cylinder.